Most low end audio consumer electronics use digital to analogue converters with fixed sampling rates because of cost reasons. But when multimedia enabled devices may be used in order to support different kinds of audio sources, the process of resampling is unavoidable, because media files might be encoded using different sampling rates, and also communication codecs use different sampling rates. Choosing different sample rates is an important matter in regard to the operating points of different audio codecs and processing methods. The more different sample rates that need to be supported, the more complex is the sample rate adaption and resampling task.
For example in the current MPEG-D USAC (USAC=Unified Speech and Audio Coding) reference model, some uncommon (not an integer multiple of 16000 Hz or 22050 Hz) sampling rates are employed. These rates are the result of a compromise between two aspects: First, a nominal sampling rate of the integrated ACELP coding tool to which it was specifically designed and which, to a degree, dictates the overall system sampling rate, and second, the desire to increase the sampling rate together with bit rate to be able to code greater audio bandwidth and/or to realize scalability.
Partly, the uncommon sampling rates are also a legacy from the AMR-WB+ system which parts of the reference model have been deduced from. Also, as common in practice in low bit rate audio coding, the sampling rate and thus the audio bandwidth are being greatly reduced at low bit rate USAC operating points.
At low USAC bit rates in particular the currently employed sampling rates exhibit both of the above mentioned problems. They are not compatible with low-cost hardware D/A converters and would involve an additional post-resampling step. Audio bandwidth is limited to the Nyquist frequency, which is well below the upper limit of the human audible range.
To adapt the output sampling rate of an audio processing unit, additional resampling functional modules are being used for this purpose, requiring a significant amount of additional computational resources. The technology used for this purpose has not changed in a lot of time, consisting basically of an interpolator and optional up sampler and down sampler modules.